The present disclosure relates to exhaust gas-driven turbochargers.
Increasingly stringent emissions regulations have driven many changes in diesel engines, one of which is the widespread use of exhaust gas recirculation (EGR), wherein exhaust gas from the engine is used as a working fluid diluent to reduce peak combustion temperatures. The formation of oxides of nitrogen (NOx) increases exponentially with temperature (eT), so a reduction of the peak temperature through heat transfer to a non-combustible gas mixed with the combustible fuel and air is very effective at reducing NOx.
In the U.S., EPA regulations have reduced NOx limits from 8 g/kW-hr in 1998 to 0.2 g/kW-hr in 2010, a 97.5% reduction in 12 years. Unfortunately, there has not been a remarkable device (such as the 3-way catalyst was for gasoline engines) invented for diesel engines. Alternative combustion modes, EGR, Selective Catalytic Reduction (SCR), and Lean NOx Traps (LNT) are four very effective methods to reduce NOx. Unfortunately none are much more than 90% effective, and therefore a combination of two or more of these technologies is generally used.
EGR has been a mainstay through the first decade of the 21st century to achieve NOx reduction in diesel engines, but it also has negative effects. These include increased wear and corrosion in the engine, contamination of the oil with soot and acidic materials, increased heat rejection of the engine through heat exchangers instead of through the exhaust, and increased pumping loss to the engine. Careful product development has been able to mitigate much of these negative effects, with the exception of increased pumping loss. The development described in the present application is aimed in part at minimizing the increased pumping loss for extremely high EGR engines, or increasing the maximum possible EGR rate while maintaining a modest pumping loss.
For EGR to flow from the exhaust manifold through an EGR cooler and then to the intake manifold, the exhaust manifold must be at a higher pressure than the intake manifold. This results in “negative pumping work” as explained below.
Traditionally, high-efficiency turbochargers were capable of producing more boost pressure than the back pressure they created on the exhaust manifold in turbo-diesel engines. This produced a “positive pumping work” and added to the work output of the engine. With a high pressure loop EGR system, this must be a “negative pumping work”, which means it reduces the work output and efficiency of the engine. The more EGR that is driven, the higher this negative pumping work becomes. In addition, the EGR is additional mass flow that must be pumped through the engine. This requires additional boost pressure to increase the density of the combined air and EGR to match the volumetric flow of the engine. The turbocharger sees additional EGR flow as a reduced “fresh air” volumetric efficiency of the engine. Assuming the fresh air mass flow remains the same, the boost must increase significantly to pump the EGR through the engine as well. Since a diesel engine runs lean with 20% or more excess oxygen, the recirculated exhaust gas still contains oxygen, and hence the fresh air that must be supplied is somewhat reduced.
At modest levels of EGR (roughly up to 10%), turbocharger efficiency must be artificially reduced to assure the needed negative pressure gradient. This has been done through a variety of methods, including increasing turbine clearances, or introducing a step area reduction in the flow path.
At moderate levels of EGR (roughly 10-20%), usually it is not necessary to reduce the efficiency of the turbocharger to achieve the required negative pressure gradient, and the negative pumping work is approximately ½ bar “pumping mean effective pressure” (PMEP) or less.
At high levels of EGR (roughly 20-30%), the pumping work becomes much more severe and the turbocharger efficiencies start to fall off as the pressure ratio rises, increasing the pumping work unnecessarily. Above 25% EGR (at full load), generally two stage turbocharging becomes necessary.
At extreme levels of EGR (over 30% at full load), the mis-match of the compressor(s) and turbine(s) becomes so severe that the fuel consumption penalty caused by negative pumping work becomes intolerable and engine manufacturers choose to lower the EGR rate and rely on other NOx reduction technologies such as SCR or LNTs.
The aerodynamic performance of compressors and turbines is a function of the volume flow, rather than the mass flow, through the stage. The compressor receives ambient air to compress, while the turbine receives high-temperature and high-pressure exhaust gas to expand. As EGR is increased, the total flow through the engine (air plus EGR) is increased, and thus the boost pressure is increased. When an extremely high level of EGR is driven through the engine, the volume flow of the turbine is very small compared to the volume flow of the compressor, and hence the compressor and turbine should be significantly different sizes. Because the speed at which the compressor and turbine desirably should be run for best efficiency is inversely proportional to the diameter of each device, the compressor and turbine therefore should operate at different speeds. With a single shaft connecting them, however, this is physically impossible. Accordingly, generally the turbine is forced to operate at a speed required by the compressor to produce the pressure ratio and mass flow required by the engine, which results in poor turbine efficiency because it is operating too slowly.
It can be shown that to achieve optimum turbine efficiency, the ratio of the turbine tip speed, U (angular rotational rate multiplied by the radius at the tip) to the gas speed, Co, must be approximately 0.7. Since the gas velocity is fixed by the engine condition, the independent variables are the turbine speed and diameter. The compressor sets the shaft speed; therefore, the only independent variable left is the turbine diameter.
Unfortunately, the turbine diameter must be kept as small as possible to preserve acceptable transient response of the engine. The flow of the turbine is a function of the diameter squared, but the inertia of the turbine is a function of the diameter of the turbine diameter to the 5th power. One can easily see that the optimum solution for dynamic response is to have the smallest diameter turbine that can pass the engine flow. This results in a poor U/Co, which limits the turbine efficiency.